Bone mineral density (BMD) before menopause is hypothesed to be important in providing sufficient bone mineral reserve so that bone loss associated with menopause and aging does not leave the individual compromised and at substantial risk for osteoporotic fracture. However, our recent investigation suggests that bone loss may occur prior to menopause. Therefore, we propose to examine premenopausal BMD levels and relate BMD changes to concurrently characterized hormones associated with approaching climacteric. This prospective longitudinal study can be undertaken efficiently because femoral bone mineral density already has been measured in a large cohort of premenopausal women in 1988. Specific aims are: 1.To measure BMD by dual-photon densitometry and dual x-ray densitometry in 383 women, aged 30-40 at baseline, over an 8-year period with 5 measurements (1988, 1992, 1993, 1994, and 1995). 2.If premenopausal bone loss occurs, to describe the loss in terms of age and rate for all measured bone sites including the spine, proximal femur and total body skeleton. 3.To characterize estradiol, testosterone and follicle stimulating hormone annually and simultaneously with BMD measurement and to examine the prognostic impact of changes in ovarian function on premenopausal BMD. 4.To determine if the relationships observed between measures of ovarian function and BMD remain consistent following adjustment for diet, body composition, and physical activity? Data analysis methodology will include analysis of variance, multiple variable regression (including growth curve analysis) and logistic regression. This study will contribute to the understanding of bone- dynamics in the perimenopause and help determine if there is accelerated bone loss associated with the premenopausal decline in ovarian function. It may identify that women at risk for accelerated premenopausal loss of bone mass could benefit from premenopausal administration of estrogens, calcium supplementation or exercise therapy.